Jimn is absolutely correct about those petcocks...I had them, and I always had to remove them. I would open them, and hardly any water would come out, they were alwats plugged with silt, scale, rust etc.

I have even had the drain holes on the block plugged; always remove petcock or plugs AND poke a crewdriver or awl up in the hole and make sure that there are no obstructions.